The invention relates to a magnetic resonance imaging apparatus which is provided with one or more electrical accessory devices such as, for example, RF body coils or catheters with coil elements which are intended for use during the examination of a patient or other object, as well as with a connection lead which is to be guided through an examination zone of the magnetic resonance imaging apparatus, which zone can be exposed to an RF field, and which lead is intended to connect the accessory device to a connection unit such as, for example, a power supply or control unit.
A magnetic resonance (MR) imaging apparatus is used in particular for the examination and treatment of patients. The nuclear spins of the object to be examined are then aligned by a steady main magnetic field (B0 field) and are excited by RF pulses (B1 field). The relaxation signals thus formed are exposed to gradient magnetic fields for the purpose of localization and are received in order to form in known manner therefrom an image of the tissue examined.
Essentially two types of construction can be distinguished, that is, the so-called open MR systems (vertical systems) in which a patient is introduced into an examination zone which is situated between the ends of a C-arm and hence is accessible from practically all sides, that is, also during the examination or treatment, and also MR systems which comprise a tubular examination space (axial systems) in which the patient is introduced.
RF coil systems serve for the transmission of the RF signals and the reception of the relaxation signals. In addition to the RE coil systems which are permanently built into the MR imaging apparatus, use is also made of RF body coils which can be flexibly arranged, for example, as a sleeve or pad around or on the region to be examined.
Furthermore, use is made of catheters which are introduced into the patient, for example, in order to take a sample of tissue during the imaging and which comprise a coil element, an oscillator or the like at the area of their tip for the purpose of localization in the image formed.
Accessory devices of this kind and other kinds are to be connected, via an electrical connection lead, to a connection unit, notably a power supply, a receiving device and/or a control device, which is situated outside the examination zone.
A problem in this respect is posed by the fact that the electrical field generated by the RF coil systems induces RF currents in the electrical connection lead leading to the relevant accessory device; these currents involve not only the risk of disturbances or destruction of the accessory device and the connection unit, but notably can give rise to substantial heating of the connection lead and, in the case of body coils and catheters, to burning of the patient when the leads are too close to the patient.
U.S. Pat. No. 6,284,971 discloses various coaxial cables for use in magnetic resonance imaging where the risk of burning of a user is to be avoided by a different configuration of the outer insulation of the cable. This outer insulation consists of a cylindrical inner shielding portion which encloses the conductor as well as of a segmented outer shielding portion, which portions are connected to one another. Between these shielding portions there may be situated a dielectric material having a comparatively high relative permitivity. In other embodiments conductive elements are provided at the ends of the segmented outer shielding portions, or such ends are connected to the inner shielding portion via a capacitor.
Cable structures of this kind, however, are comparatively voluminous, complex and expensive and the results that can be achieved thereby in respect of suppression of signals induced by the RF pulses are often inadequate, in particular in the case of high RF field strengths.